Total Cost Calulation Logic

[JAS-JPG]

Why are these changes needed?

These changes add logic to calculate the total cost of an entire conversation based on user-provided pricing. This allows users to see the overall AI API cost at the end of execution.

Related issue number

Model Costs and Cached Tokens #4835

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.